1. Field of the Invention
The present invention relates to a zoom lens barrel, and more particularly to a zoom lens barrel employing a surface wave motor as the drive source for power zooming or automatic focusing of a phototaking lens.
2. Related Background Art
In the field of zoom lens barrels, those employing the first lens group for focusing are being replaced, in recent years, by compact barrels of rear focusing system employing a rear lens group for focusing or inner focusing system employing an inner lens group for focusing. In these zoom lens barrels of a type utilizing a part of the zooming cam for focusing, there is incorporated a switching clutch for selecting the focusing or zooming operation.
Such conventional zoom lens barrels utilizing a part of the zooming cam for focusing have been associated with drawbacks that the barrel is complex in mechanism and is inevitably large in dimension, because of the presence of the switching clutch for the focusing and zooming operations. Also such barrels are not satisfactory in the convenience of use.
Also the recent zoom lens barrels frequently employ electrically powered zooming, which can be switched to manual zooming for fine adjustment. For this purpose there is incorporated a switching clutch for the power zooming and the manual zooming.
Because of the presence of such switching clutch for the power zooming and the manual zooming, such barrels are complex in mechanism and are not satisfactory in the convenience of use.
Furthermore, in a lens barrel which uses a surface wave motor, the rotor (movable member) and a stator (fixed member) are brought into contact with each other with large force at their contact surfaces because of the principle of the rotation of the surface wave motor. Therefore, the rotor must be rotated by the torque which is larger than the frictional force generated between the rotor and the stator in order to manually drive the photographing lens. As a method of manually driving the photographing lens with small force, a method disclosed in Japanese Patent Appln. Laid-Open No. 59-101608 has been known which is arranged in such a manner that the frictional force generated between the rotor and the stator is reduced. Another method has been known which is arranged in such a manner that the rotor and the stator are moved away from each other.
However, in the above-described methods, the rotor, which is positioned in contact with the stator in a frictional manner although the frictional force is not large, is rotated. Therefore, a problem arises in that the contact surface of the rotor and that of the stator wear out excessively. Furthermore, dust will be introduced into the space between the rotor and the stator because they are arranged to move away from each other, causing another problem to arise in that the contact surfaces to be damaged when the surface wave motor is rotated.
In order to overcome the above-described problems, a method has been disclosed in Japanese Patent Appln. Laid-Open No. 61-86718 which is arranged in such a manner that, when the mode selection device is in the manual focal point adjustment mode, the movable member and the fixed member of the surface wave motor are integrated before they are rotated in synchronization with the operation of the manual operation member so as to drive the photographing lens. Furthermore, in the automatic focal point adjustment mode, the fixed member is fixed to the lens barrel so as to drive the photographing lens when the movable member is rotated. According to the above-described method, the photographing lens can be driven while protecting the contact surface of the movable member and that of the fixed member from damage.
However, the method disclosed in Japanese Patent Appln. Laid-Open No. 61-86718 is arranged in such a manner that the mode selection device is slid so that frictional force is generated between the surface of the outer tube of the surface wave motor and that of the fixed tube in order to prevent the rotation of the surface wave motor in the fixed tube even if the lens holding tube is driven by the surface wave motor in the automatic focal point adjustment mode. In the manual focal point adjustment mode, the mode selection device is slid in the opposite direction so as to make the frictional force generated between the surface of the manual movement ring and the surface of the outer tube to be larger than the force for driving the lens holding tube. In order to achieve the above-described function switching operation, the mechanism becomes too complicated because mechanical mode selection switching is performed. Furthermore, a mechanical switching mechanism must be used in a case where a so-called go-home photographing function is performed in which an arbitrary range of photography is previously stored and the lens is driven to the stored range of photography after a photographing operation has been completed at another range of photography or a so-called manual focal point adjustment mode priority photography is performed in which instantaneous switching to the manual focal point adjustment mode is realized by rotating the manual operation ring during photographing in the automatic focal point adjustment mode. Therefore, it is very difficult to include the mechanism of quickly performing switching of the above-described functions.